Ionization vacuum gauge with x-ray shielding and ion reflecting means



Sept. 2. 1969 P. A REDHEAD 3,465,189

IONIZATION VACUUM GAUGE WITH X-RAY SHIELDING [\NI) ION REFLECTING MEANS Z Sheets-Sheet 1 Filed March 20, 1967 we Pm l Sept. 2, 1969 P. A. REDHEAD 3,455,189

IONIZATION VACUUM GAUGE WITH XRAY SHIELDING AND ION REFLECTING MEANS Filed March 20, 1967 2 Sheets-Sheet 2 FIG.3

1 UL .REDHLAO United States Patent Int. Cl. H01j 7/: 6, 13/28, 17/24 U.S. Cl. 313-7 4 Claims ABSTRACT OF THE DISCLOSURE A vacuum gauge of the gas ionization variety which has an accelerating grid cage for accelerating electrons which subsequently bombard and ionize gas molecules within a region substantially defined by the grid cage. The region is separated from a collecting electrode which collects the ionized molecules by a separation plate having a small hole therein through which the ionized molecules migrate. The plate assists in shielding the collecting electrode from the effects of soft X-rays produced by electron bombardment within the region. In proximity to the collecting electrode, a reflecting electrode is provided to focus the ionized molecules which migrate through the opening unto the collecting electrode. Because of the potential gradient between the reflecting and collecting electrodes, and their symmetrical relationship to the opening and the grid cage, desorbable ions are not focused on the collecting electrode.

This invention relates to a vacuum gauge and more particularly to an ionization vacuum gauge.

Gauges used to measure low gas pressures operate on the principle of gas ionization. Such devices usually include heated filaments or cathodes for the emission of electrons, means for accelerating free electrons to sufficiently high value of kinetic energy to ionize molecules within the gauge. The ions thus produced are then collected on a collector electrode. Bayard in his United States Patent No. 2,605,431 issued July 29, 1952, describes an ionization gauge based on the above described principles for measuring gas pressures substantially below mm of mercury (10 torr).

Generally there are two processes which limit the lowest pressure measurable by a hot filament ionization gauge. The first limiting process is the well known X-ray effect in which the limit is reached at a pressure where the positive ion current equals the photon current produced by soft X-rays. The second limiting process results from the desorption of molecules (commonly known as neutral ions) and/or positive ions by electron bombardment of adsorbed layers on the ion collector. This process is termed electron desorption. The X-ray limit of a gauge is principally determined by the electron energy, the gauge geometry and the material of the electrodes, while their surface covering has only a second order effect. The electron desorption limit is controlled by the gas adsorbed on the ion collector surface and is thus profoundly affected by gases present in the system either prior to, or during the pressure measurements.

Little attention has been given to the problem of reducing the electron desorption limit. In any particular gauge, either the X-ray or electron desorption limit may predominate depending on the gauge design and the surface conditions which prevail at the ion collector. In many instances, the electron desorption limit is the more troublesome of the two.

An object of this invention is to increase the range of pressures detectable with an ionization gauge and to overcome some of the above mentioned difiiculties. This is achieved by decreasing the X-ray limit and results in a reduction of the effects of X-ray photons inherently generated within the gauge.

A further object of the invention is to provide a relatively accurate indication of pressure (although having a somewhat lesser absolute sensitivity) of reducing the effects of electronically desorbable ions. This is accomplished by reducing the collection efiiciency for these desorbable ions (and to some degree the gas phase ions) thereby reducing the masking of the true pressure in the effects of electron desorption. As will become apparent the reduction of collection efficiency of gas phase ions has a greater effect on the desorbable ions than the gas phase ions. The word masking is used herein to express the superposition of one activity i.e. electron desorption, on another i.e. gas phase ion current, such that discrimination between them is impossible.

This is achieved by a preferential collecting means adapted to collect lower energy ions from the gas phase (gas phase ions) in preference to the higher energy desorbable ions. Furthermore, the preferential collecting means is also adapted to collect gas phase ions of low angular momentum in preference to the desorbable ions of high angular momentum.

The word nude is used herein to designate a gauge which has no air tight encapsulating envelope since such an envelope is unnecessary where the gauge is used in outer space or within an evacuated system.

Accordingly, the invention relates to an ionization vacuum gauge in which desorbable ions and X-ray flux are generated, while gas molecules are produced within a region, said gauge comprising:

(a) Ionization means for ionizing gas molecules;

(b) Collecting means for collecting ionized gas molecules; and

(c) Shield means disposed between said means (a) and (b) for reducing the X-ray flux incident on said collecting means, whereby the gauge is rendered less sensitive to the effects of X-ray flux and relatively more sensitive to ionized gas molecules collected on the collecting means.

The invention further contemplates that the collecting means and shield means be adapted for collecting the gas phase ions in preference to desorbable ions.

The invention will now be described by way of example, reference being made to the accompanying drawings which show two embodiments of the invention, and wherein:

FIGURE 1 is a partly sectioned elevation view of an ionization vacuum gauge constructed in accordance with the invention;

FIGURE 2 is a partly sectioned perspective view of another embodiment of the invention; and

FIGURE 3 is a cross-sectional schematic representation of the gauge of FIGURE 1.

Referring to FIGURE 1, one form of gauge includes, a glass envelope 1 provided with a lead 2 which enters the envelope 1 through a nipple 3. The lead 2 is connected to a conductive film of tin oxide (not shown) covering the interior surface of the envelope 1. The conductive film is connected through the lead 2 to a source of positive potential such that stray electrons which strike the film may be collected and conducted to the source of potential in a manner which will become apparent.

A helix 4 formed of conductive wire is mounted on conductive support rods 5 which are secured to conductive standards 6 extending through a glass base 7 of the envelope 1 to a source of potential (not shown). The

lower end of the helix 4 terminates at a ring 8 while the upper end of the helix 4 terminates at a ring 9. A wire screen (not shown) extends across the lower ring 8. The combination of helix 4, rods 5 and rings 8 and 9 form a grid cage 27. The grid cage 27 when placed at an appropriate potential, which will become apparent, is the accelerator for electrons generated by a generally U- shaped filament 10 which may be tungsten, or, preferably, thoria coated tungsten. The filament 10 is disposed within helix 4 symmetrical about the longitudinal axis thereof. The ends of the U-shaped filament 10 are embedded in support posts 11 which extend through base 7 to a source of potential (not shown). A thin wire modulator 12 coaxial with the longitudinal axis of helix 4, and preferably formed of tungsten, is supported on a modulator post 13 extending through base 7 to a switching means (not shown) for switching the potential thereof between two levels for reasons which will become apparent.

A hemispherical shaped ion reflector 14 is disposed above the ring 9 coaxial with the longitudinal axis of the helix 4 and modulator 12. The ion reflector 14 is mounted on a plurality of insulators 15 located about the periphery of the reflector 14. Reflector 14 is also supported by the upper ends of the conductive rods 5. As a result, the reflector 14 is maintained at the same potential as the grid cage 27.

Interposed between the ion reflector 14 and the grid cage 27 is a shield 26 supported by conducting rod extending through base 7 to a source of potential (not shown). The shield 26 includes a base 16 with aperture 17 therein and a side Wall 18 extending upwards from the margin of base 16. The axes of the shield 26 and the aperture 17 are aligned with the longitudinal axis of helix 14. A wire collector 20 projects through a hole in the top of the ion reflector 14 into the chamber formed by the ion reflector 14 and the base 16 of the shield 26. The collector 20 is also coaxial with the longitudinal axis of helix 4. The collector 20 is embedded into a conductive rod 21 about which is wrapped an insulating sheet 22 having an enlarged end 23 which frictionally engages a cylindrical insulating connector 19 secured to the top of the ion reflector 14 to thereby maintain the collector 20 on the axis of helix 4. The conductive rod 21 extends through the glass envelope 1 to a source of potential (not shown). The envelope 1 further includes an outlet 24 which is connected to a vacuum system (not shown) for measuring gas pressure therein.

Referring to FIGURE 2, there is illustrated another embodiment of the invention similar to that of FIGURE 1 except that the encapsulating envelope is omitted. The embodiment of FIGURE 2 is a nude ionization vacuum gauge and includes a base through which extends conductive support rods 31 and 35. Rods 31 support a grid cage 44 including a helix 32 and upper and lower rings 33 and 34. Conducting wires 38 extend across the upper support ring 33. A filament 36 which may be formed of tungsten, but preferably thoria coated tungsten, extends around the helix 32 adjacent to its lower end and is supported by rods and wire 37 embedded in base 30. The ends of the filament 36 are connected to conductive rods 35 extending through base 30 for connecting to a source of potential (not shown). A bar 39 interconnects the supports 31 for supporting a hemispherical, conductive ion reflector 40. A wire collector 41 projects through a hollow neck 40A of the reflector 40 into a cavity formed by the ion reflector 40. The collector 41 is connected to a conductive lead 42 such as by welding. The lead 42 extends through the base 30 to a source of negative potential (not shown). A conductive shield 45 including a top 46 is supported by a conductive member 47 extending through base 30 to a source of potential (not shown). The top 46 is provided with an aperture 48 the axis of which is coincident with the projection of the longitudinal axis of the helix 32. The top 46 is provided with a downwardly extending wall 49 which surrounds the upper end of the reflector 40.

A wire modulator 50 is located at the upper end of grid cage 44, and extends into the grid cage 44 along the axis thereof. The modulator 50 is embedded in a con ductive wire 51 for connecting to a switching means (not shown) such that the potential on the modulator 50 can be that of collector 41 or that of the grid cage 44. The modulator wire 50, aperture 48, neck 40A and collector 41 are coaxial with the longitudinal axis of grid cage 44.

Referring to FIGURE 3, there is shown a grid cage 55, an ion reflector 56, and a shield plate 57 disposed in approximately the same relationship to each other as the grid cage 27, ion reflector 14, and shield 26 of FIGURE 1. A collector 58 embedded in a conductor 59 projects through a hole in the shield plate 57, and a wire modulator 60 is embedded in a conductive member 61 at the opposite end of cage 55 from collector 58 to complete the gauge. The following table gives the examples of the preferred dimensions in millimeters of the parts illustrated in FIGURE 3.

Radius of helix (r l3 Radius of aperture in shield plate (r 5 Radius of collector (r 0.0875

The distance the modulator projects into grid cage (A) 5 Length of the grid cage not shielded by the shield cage (X 27 Total length of the grid cage (L 32 Distance between grid cage and shield plate 1) 5 Distance between shield plate and the tip of the collector (d 7 Distance the collector projects through ion reflector into the cavity (L 5 All components of the gauge except the grid cage and the filament are vacuum fired before assembly. The grid cage, shield plate and ion collector are outgassed by electron bombardment, as for example, 250 milliamperes at 1 kilovolt after the components have been enclosed in a glass envelope.

In operation, the gauge of FIGURE 1 is incorporated in a vacuum system by connecting the outlet 24 thereto. The filament 10 is heated by the passage of a current therethrough and thereby emits electrons. The electrons are accelerated by the grid cage 27 placed at a positive potential relative to filament 10. As a result of the open construction of the grid cage 27, few, if any, are collected thereby. The accelerated electrons bombard gas molecules Within the gauge to ionize the molecules. The positive ions created are attracted to the negative shield 26 but most of them, in the order of pass through the aperture 17 and are focused, by the positive potential on the ion reflector 14, on the grounded collector 20. The number of ions passing through the aperture 17 of the shield 26 and collector on the collector 20 may be altered as much as 50% if the potential on the wire modulator 12 is switched from ground (the potential of the collector 20) to that of the positive grid cage 27. Suitable examples of the operating conditions are: volts, grid cage 27 to filament 10; +200 volts (with respect to the grounded collector 20) for each of the filaments 10, the conductive coating (not shown) on the inner surface of the envelope 1 and the shield 26. The collector is grounded and an electron current of less than 1 milliampere is maintained.

The gauge of FIGURE 2 functions in substantially the same manner as that of FIGURE 1. Electrons are generated by the heated filament 36 placed at a positive potential with respect to the grounded collector 41. The electrons emitted by the filament 36 are accelerated by the potential on the grid cage 44. Because of the open construction of the grid cage 44 the electrons pass through into the interior of the helix 44 and bombard gas molecules therein. The positive ions thus produced are accelerated by the shield 46 which is at the same potential as the filament 36. The majority of the ionized molecules pass through aperture 41 and are reflected by the potential on the ion reflector 40, which is the same potential as the grid cage 44, to the grounded collector 41 where they are collected.

The modulator 50 which is connected to a switching means (not shown) through conductive post 51 is normally grounded so that it is at the same potential as the collector 41. If it is desired to reduce the number of ionized molecules to be collected by the collector 41, the modulator 50 is connected through the switching means to a source of potential whereby the modulator is given the potential of the grid cage 44. As a result, the number of ions impinging upon collector 41 is reduced in the neighborhood of 50%. Suitable amplifiers (not shown) and indicating means (not shown) are connected to each of the conducting posts 21 and 42 of FIGURES 1 and 2 respectively to measure the gas ion current which is a function of the number of molecules within the gauge and hence the gas pressure therein.

A gauge assembled in accordance with the above eX- ample of FIGURE 3 and evacuated using a cold cathode magnetron pump to a pressure of at least 3 10 torr is sealed off. The sealed off gauge is immersed in liquid nitrogen to further reduce the pressure therein and is operated in accordance with the example operating conditions, but with an electron current of l milliampere. The lowest indicated pressure is 7X10 torr which agrees reasonably with the theoretical calculated limit of 3 X 10" torr.

As will be apparent to those skilled in the art the coaxial arrangement of the elements of the gauge, especially of the ion collection, ion reflector and shield plate with grid cage, contribute to the gauges ability to discriminate between desorbable ions and gas phase ions as may be explained as follows: desorbable ions which are produced at or near the surface' of the grid cage, have not only a higher energy level but also a higher angular momentum than those of gas phase ions produced within the grid cage. Of the desorbable ions which pass through the aperture in the shield plate, some will strike the collector but more probable, because of their higher angular momentum, most will orbit about the collector and since the ion reflector is at the same potential as the grid cage these ions are energetically capable of striking the ion reflector as most do. Of the gas phase ions which pass through the aperture in the shield plate, most will strike the collector but some with higher angular momentum will orbit about the collector but since the ion reflector is at the same potential as the grid cage these orbiting ions are energetically incapable of overcoming the potential gradient between the ion reflector and collector; as a result, they orbit until they loose sufficient energy to be collected by the collector.

It will also be apparent to those skilled in the art that the shield shields the collector from X-rays caused by free electrons striking electrode surfaces such as the interior tin film coating on the glass envelope. This shielding eflect is enhanced by the side wall of the shield i.e., elements 18 in FIGURE 1, and 49 in FIGURE 2.

What I claim is:

1. An ionization vacuum gauge in which desorbable ions and X-ray flux are generated, while gas molecules are produced within a region, said gauge comprising:

(a) ionization means for ionizing gas molecules;

(b) collecting means for collecting ionized gas molecules; and

(c) shield means disposed between said means (a) and (b) for reducing the X-ray flux incident on said collecting means, whereby the gauge is rendered less sensitive to the effects of X-ray flux and relatively more sensitive to ionized gas molecules collected on the collecting means; and

(d) reflecting means disposed around said collecting means, said reflecting means adapted to reflect said gas ions toward said collecting means.

2. The gauge of claim 1 including means for placing said reflecting means at a potential higher than the kinetic energy of the ionized gas molecules but not higher than the kinetic energy of the desorbable ions.

3. The gauge of claim 1 wherein the shield means is a plate having an aperture therein, the reflector being larger than said aperture and hemispherical in shape such that a cavity is defined by said plate and said reflector within which cavity said collector is disposed subtending said region through said aperture.

4. The gauge of claim 1 wherein the shield means is a plate having an aperture therein, the reflector being larger than said aperture and hemispherical in shape such that a cavity is defined by said plate and said reflector within which cavity said collector is disposed subtending said region through said aperture.

References Cited UNITED STATES PATENTS 3,274,326 9/1966 Morris et al. 313-7 X 3,292,078 12/1966 Herzog 32433 3,319,117 5/1967 Wheeler 3137 X 3,341,727 9/1967 Schuemann 324-33 X 3,378,712 4/1968 Lafferty 32433 X 3,394,301 7/1968 Van Oostrom 32433 JAMES W. LAWRENCE, Primary Examiner C. R. CAMPBELL, Assistant Examiner US. Cl. X.R. 

